首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   26490篇
  免费   1263篇
  国内免费   2186篇
林业   2995篇
农学   1996篇
基础科学   3747篇
  3642篇
综合类   12678篇
农作物   775篇
水产渔业   821篇
畜牧兽医   1854篇
园艺   530篇
植物保护   901篇
  2024年   117篇
  2023年   344篇
  2022年   545篇
  2021年   665篇
  2020年   660篇
  2019年   767篇
  2018年   488篇
  2017年   784篇
  2016年   1067篇
  2015年   937篇
  2014年   1365篇
  2013年   1321篇
  2012年   2089篇
  2011年   2235篇
  2010年   1732篇
  2009年   1771篇
  2008年   1746篇
  2007年   2044篇
  2006年   1670篇
  2005年   1434篇
  2004年   1019篇
  2003年   875篇
  2002年   672篇
  2001年   578篇
  2000年   448篇
  1999年   386篇
  1998年   344篇
  1997年   290篇
  1996年   268篇
  1995年   240篇
  1994年   206篇
  1993年   194篇
  1992年   159篇
  1991年   94篇
  1990年   137篇
  1989年   85篇
  1988年   91篇
  1987年   37篇
  1986年   14篇
  1985年   4篇
  1984年   2篇
  1983年   1篇
  1981年   1篇
  1980年   1篇
  1979年   4篇
  1978年   3篇
  1977年   1篇
  1976年   1篇
  1956年   2篇
  1955年   1篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
The Danish decision support system Crop Protection Online (CPO) optimises herbicide weed control. CPO recommends specific herbicide solutions to achieve a required level of control. The aim is to apply herbicides as little as possible but as much as necessary. CPOWeeds is a version of CPO adjusted to conditions in North-eastern Spain. The predicted efficacies and the yield obtained with CPOWeeds were validated in winter cereal field trials from 2010 to 2013. All CPOWeeds treatments were related to the efficacies obtained with standard herbicide treatments decided upon by local advisors. The predictions from CPOWeeds were compared to the actually achieved efficacies in the field trials for the nine weed species at different developmental stages and for 84.2% of the comparisons the obtained efficacies were equal to or higher than predicted. The average difference between predicted and observed efficacies was 2.35 percentage points. Yield was measured in three trials and the recommendations from CPOWeeds were maintaining yield. There were two situations where CPOWeeds were performing suboptimal. One is in the early weed growth stages, as the model is not yet prepared to account for water stress on root action herbicides applied at 10-11 BBCH. The second situation was in fields with a prior unidentified population of resistant Alopecurus myosuroides. For key species in winter cereals in Spain, such as Avena sterilis, Lolium rigidum and Papaver rhoeas, CPOWeeds achieved a satisfactory control level. It was concluded that the use of CPOWeeds allowed optimisation of the herbicide application with a very high robustness. The recommendations were satisfactorily for the conditions of the Northeast of Spain and have the potential to decrease the amount of applied herbicides by at least 30%. Therefore, it can be an important tool in Integrated Weed Management.  相似文献   
172.
根据农产品供应链理论和全面质量管理理论,采用文献检索、实地调研和比较分析的方法,建立有机柑桔的生产加工质量追溯体系的技术指标标准,包括产品来源追踪的产品批号和质量安全问题原因追踪的诸多因素,确保有机柑橘质量安全。  相似文献   
173.
Quantification of currently attainable yield and fertilizer requirements can provide detailed information for assessing the food supply capacity and offer data support for agricultural decision-making. Datasets from a total of 5 408 field experiments were collected from 2000 to 2015 across the major wheat production regions in China to analyze the spatial distribution of wheat yield, the soil nutrient supply capacity(represented by relative yield, defined as the ratio of the yield under the omission of one of nitrogen(N), phosphorus(P) and potassium(K) to the yield under the full NPK fertilizer application), and N, P and K fertilizer requirements by combining the kriging interpolation method with the Nutrient Expert Decision Support System for Wheat. The results indicated that the average attainable yield was 6.4 t ha~(-1), with a coefficient of variation(CV) of 24.9% across all sites. The yields in North-central China(NCC) and the northern part of the Middle and Lower reaches of the Yangtze River(MLYR) were generally higher than 7 t ha~(-1), whereas the yields in Southwest China(SWC), Northeast China(NEC), and the eastern part of Northwest China(NWC) were usually less than 6 t ha~(-1). The precentage of area having a relative yield above 0.70, 0.85, and 0.85 for N, P, and K fertilizers accounted for 52.3, 74.7, and 95.9%, respectively. Variation existed in N, P, and K fertilizer requirements, with a CV of 24.8, 23.9, and 29.9%, respectively, across all sites. More fertilizer was needed in NCC and the northern part of the MLYR than in other regions. The average fertilizer requirement was 162, 72, and 57 kg ha~(-1) for N, P_2O_5, and K_2O fertilizers, respectively, across all sites. The incorporation of the spatial variation of attainable yield and fertilizer requirements into wheat production practices would benefit sustainable wheat production and environmental safety.  相似文献   
174.
Geographic variations in the dolphin whistles could be useful in assessing association and isolation among populations. Whistle of free-ranging Indo-Pacific humpback dolphins (Sousa chinensis) among the Pearl River Estuary (PRE), Leizhou Bei (LZB) and Sanniang Bay (SNB) populations were investigated. A total of 2850 whistles with legible fundamental contour were extracted and 15 acoustic parameters were measured. Contrary to SNB, PRE and LZB had the same relative proportion of tonal type compositions with flat and sine representing the most frequent types. The generalized linear model analysis showed significant acoustic difference among populations and tonal types. All frequency parameters in SNB were significantly higher than those in PRE and LZB, where no significant variation was observed in most of the parameters either at the population level or within each tonal type. Canonical discriminant functions analysis showed a smaller difference between PRE and LZB than between PRE and SNB and between LZB and SNB. Compared with previous recordings, recent recordings demonstrated a consistent pattern of becoming higher in whistle frequency parameters in both LZB and SNB populations, suggesting that noise pollution in LZB and SNB increasing with time according to the acoustic niche hypothesis. Dolphin whistle's geographic variations could be shaped by the combined function of the geographical barrier function of the Qiongzhou strait and local ambient noise. Considering the isolated condition and the relatively smaller population size of the humpback dolphin in the SNB, more effective and proactive conservation actions should be taken to prevent the extinction of small populations.  相似文献   
175.
福建南平市竹产业标准体系实施、评价与改进建议   总被引:2,自引:2,他引:0  
推动竹产业高质量发展,标准化工作任重道远,推进标准体系有效实施、开展标准体系评价、动态优化标准体系必不可少。本文以南平市竹产业为例,探索竹产业标准体系实施、评价与改进的方法,以期逐步建立福建省竹产业标准体系实施与评价体系。  相似文献   
176.
We evaluated the re-establishment of an Italian ryegrass pasture by self-seeding on a no-till integrated crop-livestock systems (ICLS) in the southern region of Brazil. This work is part of a long-term experimental protocol initiated in 2003. We tested the effects of various management practices, such as summer crop systems (soybean vs. maize–soybean rotation), stocking methods (continuous vs. rotational) and grazing intensities (low vs. moderate), on Italian ryegrass pasture establishment. In addition, we tested resilience of the system by testing pasture's ability to re-establish following a year without seed head production. The experiment consisted in the rotation, on the same area, of Italian ryegrass pasture grazed by sheep during the winter and up to the end of the grass production cycle, and soybean or soybean-maize grain crops rotation cultivated during the summer. The pasture established itself by self-seeding since 2005. Data were collected in 2011 and 2012 stocking season. The soybean summer crop, continuous stocking and low grazing intensity, all positively affected the production of reproductive tillers in 2011. Grazing intensity in 2011 strongly influenced early vegetative tiller densities (before crop harvest) in 2012. However, none of the grazing intensity or the stocking method treatments affected herbage mass at the end of pasture establishment in 2011 or 2012. On the other hand, the soybean summer crop positively affected pasture establishment, both in term of tiller densities and herbage mass at the end of pasture establishment. The removal of all seed heads in 2011 (preventing seed production) resulted in the total failure of pasture establishment in 2012. Overall, Italian ryegrass establishment by self-seeding relies on the annual replacement of the soil seed bank. This experiment demonstrated that under various stocking methods, moderate grazing intensity and maize or soybean summer crop, Italian ryegrass pasture establishment by self-seeding remains successful even when the stocking periods extended up to the end of the grass production cycle. Self-seeding with moderate grazing intensity ensures successful pasture establishment, reduces labour and costs and allows to increase the stocking period and so animal live weight gain over the grazing season.  相似文献   
177.
178.
新式墙面绿化在广州和深圳的应用研究   总被引:2,自引:0,他引:2  
新式墙面绿化对日益缩减的城市绿化环境至关重要,通过实地调查广州和深圳30个新式墙面绿化场所,分析其种植形式、植物种类、植物配置与景观效果,总结出深圳和广州新式墙面绿化的植物应用现状和景观效果,并讨论其发展前景。  相似文献   
179.
通过建立理论模型,并用2014年湖南省5县区20个城乡社区的调查数据,从家务劳动时间的视角分析湖南劳动力市场的性别工资差异及其影响因素。结果发现,家务劳动时间是影响湖南劳动力市场性别工资差异的重要因素,同时其他影响因素还有月工资收入、受教育程度、行业等,但职务职称对性别工资差异的影响存在不确定性。因此,政府应大力发展家庭服务业,建立健全覆盖城乡的家庭服务体系,基本满足家庭的服务需求,使女性从家务劳动中解脱出来,同时应通过政策倾斜确保女性培训比例,增强就业竞争实力;规范劳动合同,维护私有及民营企业女性从业者在劳动就业、社会保障等方面的合法权益。  相似文献   
180.
实地调研江西省新型农业社会化服务体系建设现状,对江西省目前的新型农业社会化服务体系的特点与运行模式进行分析,针对当地在服务体系构建过程中出现的问题,提出新时期的发展建议。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号